Vibronic effects in geometry and stereochemistry of metalloporphyrins and hemoproteins

A general discussion of the origin and mechanism of some peculiar geometries - small nuclear displacements - distortions from high symmetry configuration in 3dⁿ metalloporphyrins (MeP) and hemoproteins as due to the pseudo Jahn-Teller effect (PJTE), is given. It is shown that the out-of-plane position of some of the metal atoms in the free MeP is due to the vibronic mixing of he a_1g (mainly metal d_z2) MO state with the a_2u (porphyrin) one under the metal nuclear out-of-plane displacement, resulting in the PJT softening of its in-plane configuration. For the 3dⁿ metals the order of magnitudes of these softenings follows the series: Mn ≳ Fe > Co > Ni, Cu, Zn, and only the former two of them obey the required inequality leading to their in-plane configuration instability, the latter being thus quite stable (in analogous molecules, phtalocyanines, the PJT softening is much smaller). The return of the iron atom into the in-plane position in hemoglobin (Hb) by oxygenation is explained as due to the strong decrease of the PJT softening, and the appropriate changes in the spin states are shown to be consistent with this mechanism. The ability of Hb (and other hemoproteins) to coordinate small ligands follows the same 3d metal consequence as the PJT softening. The origin of the different geometries of NO, CO and O₂ coordination of MeP's and hemoproteins is shown to obey similar PJTE regularities: the stronger is the PJT mixing of the appropriate e and a₁ MO states by ligand bending, the more soft (or even unstable) are the initial high symmetry coordination geometries.